1. Field of the Invention
The present invention generally relates to semiconductor substrate processing systems. More specifically, the present invention relates to a method and apparatus for controlling the calibration cycle of a metrology tool.
2. Description of the Related Art
Ultra large scale integrated (ULSI) circuits may include more than one million micro-electronic devices (e.g., transistors, capacitors, interconnecting lines, and the like) that are formed on a substrate (e.g., a silicon (Si) wafer) and which cooperate to perform various functions within the device. Fabrication of the electronic devices includes processes in which one or more layers of a film stack of such a device are deposited or etched, thereby forming one or more structures of the device being fabricated.
During manufacturing processes, topographic dimensions of structures formed on the substrates are measured to verify that the substrate processing reactors are operating within desired ranges that facilitate high yield and productivity. One typical procedure comprises measuring the smallest widths of the structures, such as lines, columns, openings, spaces between the lines, and the like. Such widths are known as “critical dimensions”, or CDs, and are generally the most difficult elements of a structure to fabricate and measure. In advanced ULSI circuits, the critical dimensions generally are deep sub-micron dimensions having a nominal value of less than about 0.18 microns.
Metrology tools for performing critical dimension (CD) measurements of topographic structures require periodic calibration to ensure the measurements remain accurate. Generally, metrology tools are calibrated based on a predetermined number of measurements performed since the most recent calibration or, alternatively, based on a time duration that has passed since a previous calibration. These methods of determining when the metrology tool should be calibrated are referred to as “timed” calibrations. Such a timed calibration does not assess actual performance of the metrology tool between calibration cycles such that inaccurate tool performance (e.g., invalid or inaccurate CD measurements) may occur before a calibration cycle is begun. Since CD measurements are used to control process parameters, processing a substrate based upon an incorrect CD measurement can destroy one or more substrates. Furthermore, such “timed” calibration may cause calibration of the tool when such calibration is unnecessary. Consequently, substrate processing throughput may be unnecessarily impacted.
Therefore, there is a need in the art for a method and apparatus for anticipating when a metrology tool requires calibration such that the use of a calibration cycle can be accurately controlled.